1. Field of the Invention
The present invention relates to a power supply apparatus which includes a fuel cell.
2. Description of the Background Art
At the recent time, in portable electronic equipment such as a personal computer or an electric machine such as an electrically-driven tool, a secondary battery such as a lithium-ion battery and a nickel-hydrogen battery has been widely used as a power source. However, when such equipment is operated using a secondary battery, the equipment cannot be continuously operated long enough because of its limited battery capacity. For example, if a portable-type personal computer is operated using a secondary battery, its electric power can only be continuously supplied for four hours or so.
On the other hand, in recent years, attention has been drawn to a fuel cell which is capable of continuing to supply the power for a long time. For example, in supplying the power to a personal computer, a fuel cell is under consideration which can supply the power for twenty to forty hours without a break.
A fuel cell is configured by building in layers a single cell formed by sandwiching an electrolyte layer between an anode (−) and a cathode (+). Fuel and air are supplied to the anode and the cathode, respectively. Then, an electrochemical reaction is produced, so that electric power is generated. As such fuel, for example, hydrogen, methanol, or the like is used. As the load current rises, the output voltage of the fuel cell drops. Besides, even if the fuel supply is varied according to changes in the load current, the generated energy remains unchanged for a while after the fuel supply is varied. This time lag makes it difficult to change the generated power promptly, thus causing the output voltage to change as the load current fluctuates sharply. This raises a disadvantage in that if the power is supplied using a fuel cell to equipment subjected to large fluctuations in its load current, such fluctuations in the load current bring about changes in the output voltage so that the output voltage becomes unstable.
Therefore, a hybrid-type power supply apparatus is known which is formed by combining a power storage device responsive to a fluctuation in a load current, for example, a secondary battery, and a fuel cell (e.g., refer to Japanese Patent Laid-Open No. 2005-56764 specification). FIG. 8 is a block diagram, showing the configuration of such a hybrid-type power supply apparatus. A power supply apparatus 101 shown in FIG. 8 is configured by; a fuel cell 102; a DC-DC converter 103 which converts the electric power outputted from the fuel cell 102 into a power-supply voltage for operating a load apparatus 105 and outputs it; and a secondary battery 104 which is charged with the power-supply voltage outputted from the DC-DC converter 103.
The power supply apparatus 101 stores the power outputted from the fuel cell 102 temporarily in the secondary battery 104. Then, it supplied this power from the secondary battery 104 to the load apparatus 105. The secondary battery 104 responsive to fluctuations in the load current absorbs load-current changes, thereby helping stabilize the power supply apparatus 101's output voltage. Therefore, in the power supply apparatus 101, the fuel cell 102 is designed to supply the power continuously for a long time, and this fuel cell 102 charges the secondary battery 104. As a result, the power can be supplied for more hours, and at the same time, the output voltage becomes more stable.
Nevertheless, in the power supply apparatus 101 having the above described configuration, the secondary battery 104's output voltage as it is becomes an output voltage VL to the load apparatus 105. In terms of the secondary battery 104's output voltage, each kind of secondary battery has an intrinsic voltage. For example, in the case of a nickel-hydrogen secondary battery, it is approximately 1.2 volts per cell, while in the case of a lithium-ion secondary battery, it is approximately 4.2 volts per cell when it is fully charged by the fuel cell 102.
If the power-supply voltage necessary for the load apparatus 105 to operate is different from the intrinsic voltage of each such secondary battery 104, a plurality of such secondary batteries 104 need connecting in series so that the output voltage VL is set to a desirable voltage.
However, the output voltage per cell of a secondary battery is equivalent, as described above, to the intrinsic voltage of each kind of secondary battery. Hence, the output voltage VL can be set only to multiples of the output voltage intrinsic to a secondary battery. Therefore, if the operation power-supply voltage necessary for the load apparatus 105 is a voltage value different from the multiples of the output voltage intrinsic to a secondary battery, a disadvantage arises in that such a hybrid-type power supply apparatus as the power supply apparatus 101 cannot be used as a power source for the load apparatus 105.